P5. Abstract Retroviral replication depends on integration of reverse transcribed viral DNA into a host cell chromosome. This process is catalyzed by integrase (IN) in the context of a stable synaptic complex, known as the intasome. The intasome is the target for IN strand transfer inhibitors (INSTIs), the only class of HIV IN antagonists currently approved to treat HIV/AIDS. Recent years saw characterization of the intasomes from several retroviral species, collectively elucidating the conserved intasomal core assembly (CIC) responsible for integration. We have now established the first lentiviral intasome model based on maedi-visna virus (MVV), which affords detailed structural studies of lentiviral integration in the absence of solubilizing or hyperactivating mutations in IN. The preliminary study of the MVV intasome by single particle cryo-EM in the Cherepanov laboratory revealed that it is much larger than its non-lentiviral counterparts, comprising a hexadecamer (tetramer-of-tetramers) of IN. In the proposed project we take advantage of the new system to study lentiviral DNA integration into chromatin and the role of the host cell factor LEDGF/p75 in this process. As a translational component of this project, we will develop a lentiviral intasome model for structural studies of INSTIs and the mechanisms of drug resistance.